Antigen

antigen

[an´tĭ-jen]

any substance capable, under appropriate conditions, of inducing a specific immune response and reacting with the products of that response; that is, with specific antibody or specifically sensitized T lymphocytes, or both. Antigens may be soluble substances, such as toxins and foreign proteins, or particulates, such as bacteria and tissue cells; however, only the portion of the protein or polysaccharide molecule known as the antigenic determinant combines with antibody or a specific receptor on a lymphocyte. Abbreviated Ag. See also immunity. adj., adj antigen´ic.

allogeneic antigen one occurring in some but not all individuals of the same species, e.g., histocompatibility antigens and human blood group antigens; called also isoantigen.

antigen-antibody reaction the reversible binding of antigen to homologous antibody by the formation of weak bonds between antigenic determinants on antigen molecules and antigen binding sites on immunoglobulin molecules.

cancer antigen 125 (CA 125) a glycoprotein antigen found in normal adult tissues such as the epithelium of the fallopian tubes, the endometrium, the endocervix, the pleura, and the peritoneum. Elevated levels are seen in association with epithelial ovarian carcinomas, particularly nonmucinous tumors, as well as with some other malignancies, various benign pelvic disorders, tuberculosis, and cirrhosis.

carcinoembryonic antigen (CEA) an oncofetal glycoprotein antigen originally thought to be specific for adenocarcinoma of the colon, but now known to be found in many other cancers and some nonmalignant conditions. Its primary use is in monitoring the response of patients to cancer treatment.

CD antigen any of a number of cell-surface markers expressed by leukocytes and used to distinguish cell lineages, developmental stages, and functional subsets. Such markers can be identified by specific monoclonal antibodies and are numbered CD1, CD2, CD3, etc. (for cluster designation, according to how their specificity characteristics group together when analyzed by computer).

CD4 antigen an antigen on the surface of helper T cells; the normal range of helper cells is 800 to 1200 per cubic mm of blood. The human immunodeficiency virus binds to this antigen and infects and kills T cells bearing this antigen, thus gradually destroying the body's ability to resist infection. CD4 can be administered in a soluble form to increase the amount of it in the circulation and interfere with the ability of HIV to affect CD4 antigens on the cell.

class I a's major histocompatibility antigens found on virtually every cell, human erythrocytes being the only notable exception; they are the classic histocompatibility antigens recognized during graft rejection.

conjugated antigen antigen produced by coupling a hapten to a protein carrier molecule through covalent bonds; when it induces immunization, the resultant immune response is directed against both the hapten and the carrier.

cross-reacting antigen

1. one that combines with antibody produced in response to a different but related antigen, owing to similarity of antigenic determinants.

2. identical antigens in two bacterial strains, so that antibody produced against one strain will react with the other.

extractable nuclear a's ENA; protein antigens, not containing DNA, that are extractable from cell nuclei in phosphate-buffered saline; anti-ENA antibodies are a component of the antinuclear antibodies occurring in systemic lupus erythematosus and other connective tissue diseases.

Forssman antigen a heterogenetic antigen discovered in guinea pig tissues, capable of lysing sheep erythrocytes in the presence of complement. It is found usually in animal organs but occasionally in blood, and induces formation of an antibody (Forssman antibody, a type of heterophile antibody) only when combined with protein or hog serum. Davidsohn's Differential Test was historically used to differentiate between the heterophile sheep agglutinins in human serum that were due to Forssman antigen and those due to infectious mononucleosis; this is based upon the fact that boiled guinea pig kidney will absorb heterophile sheep cell agglutinins produced by Forssman antigen, but not those produced by infectious mononucleosis.

H antigen (Ger. Hauch, film), the antigen that occurs in the flagella of motile bacteria.

hepatitis B core antigen (HBcAg) a core protein antigen of the hepatitis B virus present inside complete virions (Dane particles) and in the nuclei of infected hepatic cells, indicating the presence of reproducing hepatitis B virus. The antigen is not present in the blood of infected individuals, but antibodies against it appear during the acute infection; they do not protect against reinfection.

hepatitis B e antigen (HBeAg) an antigen of hepatitis B virus sometimes present in the blood during acute infection, usually disappearing afterward but sometimes persisting in chronic disease. Anti-HBe antibodies appear transiently during convalescence; they do not protect against reinfection.

hepatitis B surface antigen (HBsAg) one present in the serum of those infected with hepatitis B, consisting of the surface coat lipoprotein of the hepatitis B virus. Tests for serum HbsAg are used in the diagnosis of hepatitis B and in testing blood products for infectivity.

heterogenetic antigen (heterophil antigen) (heterophile antigen) one capable of stimulating the production of antibodies that react with tissues from other animals or even plants.

histocompatibility a's genetically determined isoantigens present on the cell membranes of nucleated cells of most tissues, which incite an immune response when grafted onto a genetically disparate individual and thus determine the compatibility of tissues in transplantation. Major histocompatibility antigens are those that belong to the major histocompatibility complex, which in humans contains the hla antigens. Minor histocompatibility antigens are those that can cause delayed tissue rejection.

HLA a's (human leukocyte a's) see hla antigens.

H-Y antigen a minor histocompatibility antigen present in all tissues of normal males and coded for by a structural gene on the short arm of the Y chromosome; it is thought to promote the differentiation of indifferent gonads into testes, thus determining male sex.

isogeneic antigen an antigen carried by an individual which is capable of eliciting an immune response in genetically different individuals of the same species, but not in an individual bearing it.

lymphogranuloma venereum antigen a sterile suspension of Chlamydia lymphogranulomatis; used as a dermal reactivity indicator.

M antigen a type-specific antigen that appears to be located primarily in the cell wall and is associated with virulence of Streptococcus pyogenes.

mumps skin test antigen a sterile suspension of mumps virus; used as a dermal reactivity indicator.

nuclear a's the components of cell nuclei with which antinuclear antibodies react.

O antigen (Ger. ohne Hauch, without film), the antigen that occurs in the bodies of bacteria.

oncofetal antigen a gene product that is expressed during fetal development, but repressed in specialized tissues of the adult and that is also produced by certain cancers. In the neoplastic transformation, the cells dedifferentiate and these genes can be derepressed so that the embryonic antigens reappear. Examples are alpha-fetoprotein and carcinoembryonic antigen.

organ-specific antigen any antigen that occurs exclusively in a particular organ and serves to distinguish it from other organs. Two types of organ specificity have been proposed: (1) first-order or tissue specificity is attributed to the presence of an antigen characteristic of a particular organ in a single species; (2) second-order organ specificity is attributed to an antigen characteristic of the same organ in many, even unrelated, species.

partial antigen an antigen that does not produce antibody formation, but gives specific precipitation when mixed with the antibacterial immune serum.

pollen antigen the essential polypeptides of the pollen of plants extracted with a suitable menstruum, used in diagnosis, prophylaxis, and desensitization in hay fever.

antigen presentation the presentation of ingested antigens on the surface of macrophages in close proximity to histocompatibility antigens. Some populations of T lymphocytes can only be triggered by antigens that are presented in this way. Thus macrophages play a role in inducing cell-mediated immunity.

private a's antigens of the low-frequency blood groups, so called because they are found only in members of a single kindred.

prostate-specific antigen (prostatic specific antigen) an antigen that is elevated in all patients with prostatic cancer and in some with an inflamed prostate gland.

public a's antigens of the high-frequency blood groups, so called because they are found in many persons.

sequestered a's the cellular constituents of tissue (e.g., the lens of the eye and the thyroid) sequestered anatomically from the lymphoreticular system during embryonic development and thus thought not to be recognized as “self.” Should such tissue be exposed to the lymphoreticular system during adult life, an autoimmune response would be elicited.

somatic a's antigens, usually cell surface antigens, of the body of a bacterial cell, in contrast to flagellar or capsular antigens.

T antigen

1. any of several antigens, coded for by the viral genome, associated with transformation of infected cells by certain DNA tumor viruses. Called also tumor antigen.

2. an antigen present on human erythrocytes that is exposed by treatment with neuraminidase or contact with certain bacteria.

tumor-specific antigen (TSA) any cell-surface antigen of a tumor that does not occur on normal cells of the same origin.

V antigen (Vi antigen) an antigen contained in the sheath of a bacterium, as Salmonella typhosa (the typhoid bacillus), and thought to contribute to its virulence.

xenogeneic antigen an antigen common to members of one species but not to members of other species; called also heterogeneic antigen.

an·ti·gen (Ag),

(an'ti-jen),

Any substance that, as a result of coming in contact with appropriate cells, induces a state of sensitivity or immune responsiveness and that reacts in a demonstrable way with antibodies or immune cells of the sensitized subject in vivo or in vitro. Modern usage tends to retain the broad meaning of antigen, employing the terms "antigenic determinant" or "determinant group" for the particular chemical group of a molecule that confers antigenic specificity.See also: hapten.

antigen

/an·ti·gen/ (an´tĭ-jen) any substance capable of inducing a specific immune response and of reacting with the products of that response, i.e., with specific antibody or specifically sensitized T lymphocytes, or both. Abbreviated Ag.antigen´ic

cancer antigen 125 (CA 125) a surface glycoprotein associated with müllerian epithelial tissue; elevated serum levels are often associated with epithelial ovarian carcinomas, particularly with nonmucinous tumors, but are also seen in some other malignant and various benign pelvic disorders.

capsular antigen one found in the capsule of a microorganism.

carcinoembryonic antigen (CEA) a cancer-specific glycoprotein antigen of colon carcinoma, also present in many adenocarcinomas of endodermal origin and in normal gastrointestinal tissues of human embryos.

CD antigen any of a number of cell surface markers expressed by leukocytes and used to distinguish cell lineages, developmental stages, and functional subsets; such markers can be identified by monoclonal antibodies.

class I antigens major histocompatibility antigens found on every cell except erythrocytes, recognized during graft rejection, and involved in MHC restriction.

common acute lymphoblastic leukemia antigen (CALLA) a tumor-associated antigen occurring on lymphoblasts in about 80 per cent of patients with acute lymphoblastic leukemia (ALL) and in 40–50 per cent of patients with blastic phase chronic myelogenous leukemia (CML).

complete antigen one which both stimulates an immune response and reacts with the products of that response.

conjugated antigen one produced by coupling a hapten to a protein carrier molecule through covalent bonds; when it induces immunization, the resultant immune response is directed against both the hapten and the carrier.

D antigen a red cell antigen of the Rh blood group system, important in the development of isoimmunization in Rh-negative persons exposed to the blood of Rh-positive persons.

Forssman antigen a heterogenetic antigen inducing the production of antisheep hemolysin, occurring in various unrelated species, mainly in the organs but not in the erythrocytes (guinea pig, horse), but sometimes only in the erythrocytes (sheep), and occasionally in both (chicken).

H antigen

1. a bacterial flagellar antigen important in the serological classification of enteric bacilli.

2. the precursor of the A and B blood group antigens; normal type O individuals lack the enzyme to convert it to A or B antigens.

hepatitis B surface antigen (HBsAg) a surface coat lipoprotein antigen of the hepatitis B virus, peaking with the first appearance of clinical disease symptoms. Tests for serum HBsAg are used in the diagnosis of acute or chronic hepatitis B and in testing blood products for infectivity.

heterologous antigen an antigen that reacts with an antibody that is not the one that induced its formation.

heterophil antigen , heterophile antigen an antigen common to more than one species and whose species distribution is unrelated to its phylogenetic distribution (viz., Forssman antigen, lens protein, certain caseins, etc.).

histocompatibility antigens genetically determined isoantigens found on the surface of nucleated cells of most tissues, which incite an immune response when grafted onto a genetically different individual and thus determine compatibility of tissues in transplantation.

human leukocyte antigens histocompatibility antigens (glycoproteins) on the surface of nucleated cells (including circulating and tissue cells) determined by a region on chromosome 6 bearing several genetic loci, designated HLA-A, -B, -C, -DP, -DQ, -DR, -MB, -MT, and -Te. They are important in cross-matching procedures and are partially responsible for the rejection of transplanted tissues when donor and recipient HLA antigens do not match.

H-Y antigen a histocompatibility antigen of the cell membrane, determined by a locus on the Y chromosome; it is a mediator of testicular organization (hence, sexual differentiation) in the male.

organ-specific antigen any antigen occurring only in a particular organ and serving to distinguish it from other organs; it may be limited to an organ of a single species or be characteristic of the same organ in many species.

tumor-associated antigen a new antigen acquired by a tumor cell line in the process of neoplastic transformation.

tumor-specific antigen (TSA) cell-surface antigens of tumors that elicit a specific immune response in the host.

Vi antigen a K antigen of Salmonella typhi originally thought responsible for virulence.

antigen

(ăn′tĭ-jən) also

antigene

(-jēn′)

n.

A substance that when introduced into the body stimulates the production of an antibody. Antigens include toxins, bacteria, foreign blood cells, and the cells of transplanted organs.

an′ti·gen′ic(-jĕn′ĭk) adj.

an′ti·gen′i·cal·ly adv.

an′ti·ge·nic′i·ty(-jə-nĭs′ĭ-tē) n.

antigen

[an′tijən]

Etymology: Gk, anti + genein, to produce

a substance that the immune system recognizes as foreign and mounts an immune response against. The immune response may be either production of an antibody, a cell-mediated response, or both. antigenic,adj.

antigen

A molecule that usually has a molecular weight of > 1 kD, is a protein, often foreign (i.e., non-self), and which is capable of evoking a specific immune response (antibody production).

an·ti·gen

(Ag) (an'ti-jen)

Any substance that, as a result of coming in contact with target cells, induces a state of sensitivity or immune responsiveness after a latent period (days to weeks) and that reacts in a demonstrable way with antibodies or immune cells of the sensitized subject in vivo or in vitro. Modern usage tends to retain the broad meaning of antigen, employing the terms "antigenic determinant" or "determinant group" for the particular chemical group of a molecule that confers antigenic specificity. See also: hapten Synonym(s): immunogen.

[anti(body) + G. -gen, producing]

antigen

Any molecule recognized by the immune system of the body as signalling ‘foreign’, and which will provoke the production of a specific ANTIBODY. Antigens include molecules on the surfaces of infective viruses, bacteria and fungi, pollen grains and donor body tissue cells.

antigen (Ag)

a complex molecule (usually a protein or carbohydrate) that, when introduced into the body, induces an IMMUNE RESPONSE which includes the production of specific ANTIBODIES. Antigens can be toxins (as in snake venom) or molecules on cell surfaces (e.g. A/B antigens on red blood cells).

Antigen

A substance that stimulates the immune system to manufacture antibodies (immunoglobulins). The function of antibodies is to fight off intruder cells, such as bacteria or viruses, in the body. Antigens stimulate the blood to fight other blood cells that have the wrong antigens. If a person with blood type A is given a transfusion with blood type B, the A antigens will fight the foreign blood cells as though they were an infection.

antigen,

n any substance regarded by the body as foreign that provokes an immune system response.

antigen

Any substance that can stimulate an immune response in the body and can react with the products of that response, that is, with specific antibodies or specifically sensitized T lymphocytes, or both. Antigens include bacteria, foreign substance (e.g. dust mite, grass, pollen of trees), toxins and viruses. Seeallergic reactions; sensitization.

an·ti·gen

(an'ti-jen)

Any substance that, as a result of coming in contact with appropriate cells, induces a state of sensitivity or immune responsiveness and reacts in a demonstrable way with antibodies or immune cells of the sensitized subject in vivo or in vitro.

[anti(body) + G. -gen, producing]

antigen

any substance which is capable, under appropriate conditions, of inducing a specific immune response and of reacting with the products of that response; that is, with specific antibody or specifically sensitized T lymphocytes, or both. Antigens may be soluble substances, such as toxins and foreign proteins, or particulate, such as bacteria and tissue cells; however, only a small portion of the protein or polysaccharide molecule known as the antigenic determinant or epitope is recognized by the specific receptor on a lymphocyte. Similarly the antibody or effector lymphocyte produced by the response combines only with the one antigenic determinant. A bacterial cell or large protein will have many hundreds of antigenic determinants, some of which are more important than others in protective immunity. Abbreviated Ag.

tumor-specific antigen present on the membrane of cells in cats infected with feline leukemia virus.

flagellar antigen

H antigen (below).

flea antigen

1. some components of flea saliva, as well as whole flea extracts, are antigenic and certain individuals may become hypersensitive to flea bites; the most common hypersensitivity in dogs.

2. extracts, usually of whole fleas, but sometimes of flea saliva, are used for intradermal skin testing and desensitization procedures.

Forssman antigen

heterophil antigen occurring in various unrelated species, mainly in the organs but not in the erythrocytes (guinea pig, horse), but sometimes only in the erythrocytes (sheep), and occasionally in both (chicken). Antibody to Forssman antigen is usually recognized by agglutination of sheep red blood cells.

group specific (gs) antigen

common to a certain group of organisms, e.g. streptococci, oncornaviruses.

H antigen

[Ger.] Hauch (film) the antigen that occurs in the bacterial flagella.

heterogeneic antigen

see xenogeneic antigen (below).

heterophil antigen, heterogenetic antigen

one capable of stimulating the production of antibodies that react with tissues from other animals or even plants.

hidden antigen

one not normally exposed to circulating lymphocytes, e.g. within central nervous tissue, testicular tissue and certain intracellular components, so they do not normally evoke an immune response.

a histocompatibility antigen of the cell membrane, determined by a locus on the Y chromosome; it is a mediator of testicular organization (hence, sexual differentiation) in the male.

Ia a's

histocompatibility antigens governed by the I region of the major histocompatibility complex (MHC), located principally on B lymphocytes, although T lymphocytes, skin and certain macrophages may also contain Ia antigens.

isogenic antigen

an antigen carried by an individual, or members of the same inbred strain, which is capable of eliciting an immune response in genetically different individuals of the same species, but not in individuals bearing it.

K a's

bacterial capsular antigens.

L antigen

a capsular antigen of Escherichia coli.

Ly a's

antigenic cell-surface markers of subpopulations of T lymphocytes, classified as Ly 1, 2 and 3; they are associated with helper and suppressor activities of T lymphocytes.

lymphocyte-defined (LD) a's

class II antigens found in lymphocytes, macrophages, epidermal cells and sperm. Important in graft rejection.

M antigen

a type-specific antigen that appears to be located primarily in the cell wall and is associated with virulence of Streptococcus pyogenes.

Marek's tumor-specific antigen (MATSA)

found on the surface of cells infected by Marek's disease herpesvirus.

Nègre antigen

an antigen prepared from dead, dried and triturated tubercle bacilli by means of acetone and methyl alcohol; used in serum tests for tuberculosis in humans.

nuclear a's

the components of cell nuclei with which antinuclear antibodies react.

O antigen

[Ger.] ohne Hauch (without film) the antigen that occurs in the cell wall of bacteria.

oncofetal antigen

a gene product that is expressed during fetal development, but repressed in specialized tissues of the adult and that is also produced by certain cancers. In the neoplastic transformation, the cells dedifferentiate and these genes can be derepressed so that the embryonic antigens reappear. Examples are alpha-fetoprotein and carcinoembryonic antigen.

organ-specific antigen

any antigen that occurs exclusively in a particular organ and serves to distinguish it from other organs. Two types of organ specificity have been proposed: (1) first-order or tissue specificity is attributed to the presence of an antigen characteristic of a particular organ in a single species; (2) second-order organ specificity is attributed to an antigen characteristic of the same organ in many, even unrelated species.

partial antigen

see hapten.

pollen antigen

the essential polypeptides of the pollen of plants extracted with a suitable menstruum, used in diagnosis, prophylaxis and desensitization in hay fever.

antigen presentation

the presentation of peptide derivatives of antigens on the surface of antigen presenting cells (APCs), which include macrophages, dendritic cells and B lymphocytes, in association with class II major histocompatibility complex (MHC) antigens as required for recognition by T lymphocytes. Also includes antigen presentation in association with MHC class I by cells that are targets for lysis by cytotoxic T lymphocytes.

antigens of the low-frequency blood groups, so-called because they are found only in members of a single kindred.

recall antigen

an antigen to which an individual has previously been sensitized and which is subsequently administered as a challenging dose to elicit a hypersensitivity reaction.

antigen receptors

immunoglobulin molecules on the cell membranes of B lymphocytes and a structurally related, but quite distinct molecule on the surface of T lymphocytes which recognize particular antigenic determinants of an antigen.

certain antigens, e.g. the lens of the eye and thyroid proteins, that are sequestered anatomically from the immune system during embryonic development and thus thought not to be recognized as 'self'. Should such antigens be exposed to the immune system during adult life, an autoimmune response would be elicited.

serologically defined (SD) antigen

class I antigen of the major histocompatibility complex, identifiable by the use of specific antisera.

synthetic antigen

chemically synthesized or produced by recombinant DNA technology, the synthesis of polymers, based on sequences found in microbial antigens, has been used in the production of vaccines.

T-dependent antigen

the immune response of most antigens requires T helper (Th) lymphocytes; lymphokines produced by T lymphocytes determine the characteristics of antibodies produced, which may change during the immune response.

thymus-dependent antigen

an antigen that requires T lymphocyte participation before an immune response can occur. Most antigens are of this type.

thymus-independent antigen

an antigen that elicits an antibody response without the participation of T lymphocytes. Usually large carbohydrate molecules with repeating epitopes are of this type.

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